This invention generally relates to moisture content measuring apparatus and method, and more particularly relates to a relatively inexpensive moisture content sensor station and the use of a plurality of such sensor stations to control an industrial process that is dependent on the moisture content of a plurality of different materials.
As is well known, knowledge of the moisture content of materials is desirable for many industrial processes. For example, in the process of making asphalt paving materials, sand and various grades of gravel or rocks--collectively referred to as virgin aggregate--are generally mixed with bituminous or liquid asphalt to form "hot mix" or HMA. However, the virgin aggregate must be sufficiently dry and hot or the liquid asphalt will not properly adhere to the sand and gravel. Also, the final product must be sufficiently hot such as 275-325 degrees Fahrenheit to be placed on the road bed. Therefore, in the typical operation, the virgin aggregate is introduced into one end of a large drum and heated by a burner before passing into a mixing zone where the liquid asphalt is introduced.
As is also known, it is desirable to know the initial moisture content of the virgin aggregate because that affects the number of BTUs required to dry and heat the virgin aggregate. If too little heat is applied, the virgin aggregate will not be sufficiently dried and heated; conversely, if too much heat is applied, energy is wasted and the virgin aggregate is overheated. Further, moisture content also effects the proper amount of liquid asphalt to be added because that is normally specified as ratio or percentage by weight of dry virgin aggregate (i.e. without water).
In a typical scenario, the sand, gravel, and rocks are stored in different piles or bins, and are loaded onto a common conveyor where they are mixed and fed into the large drum. Periodically, such as once or twice a day, an operator draws a sample of the virgin aggregate and carries it in a bucket to a lab where the moisture content is measured. Moisture content is generally defined as the ratio of water weight to the material weight plus the water weight. A conventional and very accurate method for determining moisture content is referred to as the water evaporation method. In such method, the sample of the virgin aggregate is first weighed, and then it is heated for a sufficient period of time to evaporate or drive off all of the moisture within the sample. Next, the sample is reweighed. The material weight plus water weight is, of course, provided by the initial weighing, and the water weight is the difference between the first weighing and the reweighing after the water has been driven off. It is noted that some states define moisture content as the ratio of water weight to dry material weight. In any event, the resulting moisture content value, typically expressed as a percentage, is then input to a control computer that makes automatic adjustments to the process. For example, if the moisture content is 5%, the computer might typically add to the heating requirements for dry aggregate an amount sufficient to drive off water weighing 5% of the total weight. Further, the computer might typically calculate that the virgin aggregate will weigh 5% less after the water has been driven off in the heating zone of the drum, and make a corresponding decrease in the liquid asphalt that is added.
The above described method has a number of disadvantages. First, it requires a substantial amount of operator time, and therefore is labor intensive and prone to errors. Further, the sampling rate is far too limited. The moisture contents of aggregate can change quickly and dramatically depending on atmospheric conditions, and the moisture content of the individual components generally change independently. Sand, for example, is very volatile in terms of moisture content; it will pick up moisture easily in a brief rain shower, but will also dry out more quickly than other materials with a little sunshine. Moisture content can also vary significantly depending on where a particular material is drawn from a stockpile. Also, another disadvantage is that it may be necessary to interrupt the flow of aggregate in order to obtain a sample.
An inaccurate initial moisture content value can also lead to another problem. In order to check for a specified ratio of liquid asphalt to virgin aggregate, an operator typically takes a sample of the finished hot-mix from the drum and carefully weighs it. Then he washes off all the liquid asphalt, dries the aggregate in an oven, and reweighs the sample. The difference between the beginning weight and the final weight is supposed to be the weight of the liquid asphalt. However, if all of the water wasn't driven off in the drum, it would be removed in this drying process, and recorded as liquid asphalt weight. This data could lead to the mistaken conclusion that the liquid asphalt content is too high, and result in an order to reduce the amount of liquid asphalt being added.
Another prior art method of determining moisture content of a material takes advantage of the fact that infrared energy is known to be absorbed by water at very specific wavelengths. That is, the absorptivity of infrared energy by water or moisture is known to be dependent on wavelength. In one commercially available system, the material is illuminated with broadband infrared energy. As is known, the reflected infrared energy power spectrum is altered according to the surface moisture on the material. For example, if the material has a relatively large amount of moisture on its surface, reflection of energy at wavelengths of high water absorption will be greatly reduced while reflections of energy at wavelengths of low absorption will be less affected by the surface moisture. It follows that if the material has relatively little water moisture, the reflected spectrum will be more uniform. In this system, a stationary light detector is positioned immediately behind a chopper wheel having a plurality of narrow band pass filters each disposed at a different angular orientation. Therefore, as the wheel rotates, the detector first sees infrared energy at wavelength .lambda.1 passing through a first filter, and subsequently sees infrared energy at wavelength .lambda.2 passing through a second filter. As a result, the detector provides a sequence of pulses having relative amplitudes that are a function of the absorption of infrared energy at the respective wavelengths by the surface moisture. In particular, one of the wavelengths .lambda.1 is not readily absorbed by surface moisture, and its pulses serve to provide a reference value related to surface parameters or characteristics of the material. The other wavelength .lambda.2 is more readily absorbed by surface moisture, and its pulses provide a measure of the surface moisture.
With the above described system, the operator initiates a calibration process wherein the ratio of pulses for .lambda.1 and .lambda.2 are stored. Then the operator takes a sample of the material and determines the moisture content of the sample by an accurate method such as the water evaporation method described above. The system has a keypad, and the operator inputs the actual moisture content for the stored ratio. The moisture content of the material can then be changed such as by wetting it with water, and then the process is repeated again building up a table of ratios and their corresponding moisture contents as actually measured. Subsequently, when the system is operating, the table is used to interpolate moisture contents for each real time ratio that is measured. In such manner, real time moisture content is electrically and automatically determined based on a correlation with previous measurements.
One problem with the heretofore described moisture sensing system is that it is relatively expensive. Further, during calibration in an industrial environment, the operator may have to carry the samples some distance to a lab to perform a water evaporation process. In addition to the labor time, trained operators are required, and errors may occur in transcribing moisture content data and entering it through the keypad of the sensor to create the table. Also, if the operational configuration of the materials changes, a complete and time consuming recalibration is generally required.